For a number of years the coating industry has been engaged in substantial developmental programs in the quest for procedures which would allow the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed commensurate with the contiguous processing steps. The industry has desired to eliminate the volatile solvents required in many of the well-known coating processes because of potential hazards or because of the cost of equipment to handle the evolved solvent vapors. In addition, the industry has been seeking coating formulations which would produce coatings which were durable and which would permit substantial additional processing of the workpiece, such as metal forming operations where the substrate is metal strip to be formed into container bodies, blanks and closures, or where plastic strip and paperboard are coated and to be formed.
Epoxy coating formulations have long been recognized as affording desirable properties in the finished coating, especially the toughness to withstand further processing. However, the problem has remained to develop a low cost epoxy coating formulation which would combine the desired rheological properties for the coating application with both reasonable pot life and rapid curing on the production line.
In Licari et al U.S. Pat. No. 3,205,157 granted Sept. 7, 1965 and Schlesinger U.S. Pat. No. 3,708,296, granted Jan. 2, 1973, there are described photopolymerizable epoxide formulations containing diazonium salts as photoinitiators which polymerize rapidly upon exposure to electromagnetic radiation to cure the formulations to provide durable coatings. In Watt U.S. Pat. No. 3,794,576, granted Feb. 26, 1974, there are described desirable epoxy formulations which combine the desired rheological properties with suitable pot life and rapid curing at ambient temperatures, by incorporation of a photoinitiator and at least about 15 percent by weight of epoxidic esters having two epoxycycloalkyl groups.
Since the disclosures of Schlesinger and Watt, a number of patents and publications have appeared proposing various photoinitiators for the epoxy formulations which could replace the diazonium catalysts specifically described in the Schlesinger and Watt Patents. Among these are the sulfonium salt catalysts disclosed in Smith U.S. Pat. No. 4,069,054 granted Jan. 17, 1978; Barton U.S. Pat. No. 4,090,936 granted May 23, 1978; and Crivello U.S. Pat. Nos. 4,069,055 granted Jan. 17, 1978 and U.S. Pat. No. 4,058,401 granted Nov. 15, 1977. Also proposed are the halonium salts described in Crivello U.S. Pat. No. 4,026,705 granted May 31, 1977.
Although the sulfonium salt compounds provide effective photoinitiators, the range of ultraviolet radiation to which the most commonly employed triphenyl sulfonium hexafluorophosphate compounds are sensitive is somewhat limited. More recently, it has been found that bis-alkyl/aryl bis-salts such as bis-[4-(diphenylsulfonio)phenyl] sulfide bis-hexafluorophosphate compounds (BDS) and related fluoroborate, fluoroarsenate, and fluoroantimonate compounds are effective photoinitiators with a broader range of spectral sensitivity in the ultraviolet range. The method for making BDS-type photoinitiators and the use thereof in the cationic polymerization of epoxy compounds are disclosed and claimed in Watt U.S. patent application Ser. No. 20,240 filed Mar. 14, 1979, and Chang U.S. patent application Ser. No. 20,514 filed Mar. 14, 1979, now U.S. Pat. No. 4,197,174, granted Apr. 8, 1980.
As is well known, it is frequently desirable to provide pigmentation in the coating for aesthetic purposes. This pigmenting material may interfere with the photopolymerization process by reason of its absorption of light waves in the activating spectral range. This is particularly acute in the case of photopolymerizable white colored compositions containing titanium dioxide as the pigmenting material.
When the more common diazonium catalysts and the more recent sulfonium catalysts are employed to cure a titanium dioxide pigmented composition, the resultant coatings exhibit a wrinkled surface appearance and frequently fail to cure adequately below the surface. It is believed that the photoinitiator at the surface, when activated by the radiation, cures the surface rapidly while the resinous composition below the surface lags far behind in curing due to the absorption of all or most of the radiation of the wavelength needed for activation. The resultant distortion of the coating between surface and body produces the wrinkled condition.
In Tarwid U.S. Pat. No. 4,054,498 granted Oct. 18, 1977, there is disclosed a photopolymerizable epoxide coating composition which develops good gloss and which contains the epoxidic esters having at least two epoxycycloalkyl groups of the aforementioned Watt Patent, substantial amounts of an aluminum oxide coated titanium dioxide pigment and 2,5-diethoxy-4-(p-tolylmercapto)benzene diazonium hexafluorophosphate. Although this photoinitiator has a spectral sensitivity which avoids excessive reduction due to the absorption of light rays by the pigment, compositions utilizing it also exhibit some sensitivity to ambient white light and a tendency to prematurely cure when so exposed.
Accordingly, it is an object of the present invention to provide a novel photoinitiatable cationically polymerizable white pigmented epoxide coating composition utilizing a novel photoinitiator component, which composition evidences greatly reduced or eliminated tendency towards premature curing under ambient white light but which rapidly cures upon exposure to activating ultraviolet radiation.
It is also an object to provide such a composition which is relatively stable during extended periods of storage prior to exposure to light and which will nevertheless polymerize rapidly upon exposure to electromagnetic radiation of below about 400 nanometers (4000 Angstroms) to produce coatings with desirable mechanical and aesthetic properties.
Another object is to provide a novel coating process using such a white coating composition and which is adaptable to a wide variety of high speed coating lines and which does not require extensive or expensive equipment to effect photopolymerization of the applied coating.